1. Field of the Invention
The present invention relates to a television tuner capable of receiving a CATV broadcasting signal in which a television broadcasting signal and an FM broadcasting signal are mixed and a ground wave FM broadcasting signal received via an FM broadcast receiving antenna (hereinbelow, simply called an FM antenna).
2. Description of the Related Art
A conventional television tuner will be described with reference to FIG. 3. In FIG. 3, a television tuner comprises a tuner unit 31 for receiving a CATV broadcasting signal and a ground wave FM broadcasting signal, an intermediate frequency circuit 32, a frequency converting circuit 33, a first demodulator 34 and a second demodulator 35.
An input change-over switch 36 is connected to the input terminal 31a of the tuner unit 31. A CATV cable 37 and an FM antenna 38 are connected to the input change-over switch 36. A CATV broadcasting signal led from the CATV cable 37 or a ground wave FM broadcasting signal received by the FM antenna 38 is received by the tuner unit 31. An output change-over switch 39 which operates interlockingly with the switching operation of the input change-over switch 36 is connected to the output terminal 31b of the tuner unit 31. To the output change-over switch 39, the intermediate frequency circuit (hereinbelow, called an IF circuit) 32 and the frequency converting circuit 33 are connected.
When the tuner unit 31 is connected to the CATV cable 37 by the input change-over switch 36, it is connected to the IF circuit 32. When the tuner unit 31 is connected to the FM antenna 38 by the input change-over switch 36, it is connected to the frequency converting circuit 33 by the output change-over switch 39.
The tuner unit 31 has a VHF circuit unit 31c and a UHF circuit unit 31d each comprising a tuning circuit, a mixing circuit, a local oscillating circuit, and the like (which are not shown). The tuner unit 31 frequency converts each of the CATV broadcasting signal and the ground wave FM broadcasting signal supplied to the input terminal 31a into an intermediate frequency signal (hereinbelow, referred to as an IF signal) and outputs the IF signal. Since the frequency of the ground wave FM broadcasting signal is close to the frequency of the low band of the VHF band in the CATV broadcasting signal, the ground wave FM broadcasting signal is received by the VHF circuit unit 31c. 
At the occasion of receiving the broadcasting signal in the VHF band in the CATV broadcasting signal or the ground wave FM broadcasting signal, the received broadcasting signal in the VHF band or ground wave FM broadcasting signal is supplied to the VHF circuit unit 31c and is frequency converted into an IF signal. Similarly, at the occasion of receiving the UHF band broadcasting signal, the received UHF band broadcasting signal is supplied to the UHF circuit unit 31d and is frequency converted into an IF signal.
The IF signal of the CATV broadcasting signal outputted from the tuner unit 31 is supplied to the IF circuit 32. For example, the IF signal in a US channel includes an audio intermediate frequency signal (having a frequency of 41.25 MHz) S and a video intermediate frequency signal (having a frequency of 45.75 MHz) P and is supplied to the first demodulator 34. The IF signal is demodulated by the first demodulator 34 into an audio signal A and a video signal V. The audio and video signals A and V are thus obtained.
In case of receiving the ground wave FM broadcasting signal, the IF signal outputted from the VHF circuit unit 31c in the tuner unit 31 is supplied to the frequency converting circuit 33. The IF signal of the ground wave FM broadcasting signal has the same frequency as that of the audio IF signal S of the CATV broadcasting signal.
The IF signal outputted from the VHF circuit unit 31c is, generally, frequency converted by the frequency converting circuit 33 to generally used 10.7 MHz and the audio signal A is obtained by the second demodulator 35 as a general demodulator for FM radio which is connected to the next stage.
Such a CATV broadcasting signal includes not only a regular television broadcasting signal having what is called a video signal but also an FM broadcasting signal having only an audio signal and no video signal. Since a conventional television tuner, however, performs demodulation into the audio signal after obtaining a beat signal between the video IF signal P and the audio IF signal S, the FM broadcasting signal in the CATV broadcasting signal having no video IF signal P cannot be received.
The invention has been achieved to solve the problem and its object is to receive a television broadcasting signal and an FM broadcasting signal in a CATV broadcasting signal and a ground wave FM broadcasting signal which are led through a cable.
In order to solve the problem, a television tuner of the invention comprises: a tuner unit for receiving a CATV broadcasting signal in which a VHF band television broadcasting signal, a UHF band television broadcasting signal, and an FM broadcasting signal are mixed and a ground wave FM broadcasting signal, frequency converting each of the VHF band television broadcasting signal, the UHF band television broadcasting signal, the FM broadcasting signal, and the ground wave FM broadcasting signal into an intermediate frequency signal, and outputting the intermediate frequency signal; an intermediate frequency circuit for generating an audio intermediate frequency signal and a video intermediate frequency signal from the intermediate frequency signal; a demodulator for demodulating the audio intermediate frequency signal and the video intermediate frequency signal; and a pseudo carrier wave oscillator for outputting a pseudo carrier wave having the same frequency as that of the video intermediate frequency signal, wherein when the tuner unit receives the FM broadcasting signal in the CATV broadcasting signal or the ground wave FM broadcasting signal, the pseudo carrier wave is supplied to the demodulator. Accordingly, both of the FM broadcasting signal and the television broadcasting signal can be demodulated by the common demodulator.
According to a television tuner of the invention, the tuner unit comprises: an antenna switch circuit having a first input terminal to which the CATV broadcasting signal is supplied, a second input terminal to which the ground wave FM broadcasting signal is supplied, and an output terminal from which the supplied CATV broadcasting signal and the ground wave FM broadcasting signal are outputted, in which a first switching diode is provided between the first input terminal and the output terminal, and a second switching diode is provided between the second input terminal and the output terminal; a VHF circuit unit which is connected to the output terminal of the antenna switch circuit, to which the VHF band television broadcasting signal, the FM broadcasting signal, and the ground wave FM broadcasting signal are supplied, and from which an intermediate frequency signal is outputted; and a UHF circuit unit which is connected to the first input terminal, to which the UHF band television broadcasting signal is supplied, and from which an intermediate frequency signal is outputted through the output terminal, wherein when the VHF band television broadcasting signal or the FM broadcasting signal is received, the second switching diode is brought out of conduction and the first switching diode is brought into conduction, and when the ground wave FM broadcasting signal is received, the first switching diode is brought out of conduction, and the second switching diode is brought into conduction. Consequently, when the VHF circuit unit receives the VHF band television broadcasting signal or the FM broadcasting signal from the cable, the FM antenna is disconnected from the VHF circuit unit by the antenna switch circuit. When the VHF circuit unit receives the ground wave broadcasting signal or when the UHF circuit unit receives the UHF band television broadcasting signal, the VHF circuit unit is disconnected from the cable by the antenna switch circuit. It can be thus prevented that the VHF circuit unit and the UHF circuit do not interfere each other at the time of reception.
According to a television tuner of the invention, a third switching diode is provided between the first input terminal and the ground, a fourth switching diode is provided between the second input terminal and the ground, the third switching diode is brought out of conduction and the fourth switching diode is brought into conduction when the VHF band television broadcasting signal or the FM broadcasting signal is received, and the third switching diode is brought into conduction and the fourth switching diode is brought out of conduction when the UHF band television broadcasting signal or the ground wave FM broadcasting signal is received. Consequently, when the first and second switching diodes are not conducting, it can be prevented that the television broadcasting signal and the FM broadcasting signal are leaked from the first and second switching diodes and inputted to the VHF circuit unit.
A television tuner of the invention further comprises a switching control unit including: a switching voltage generator having a first output terminal from which a first switching voltage for switching the VHF circuit unit into a mode of receiving a low-band television broadcasting signal is outputted, a second output terminal from which a second switching voltage for switching the UHF circuit unit into an operating mode is outputted, and a third output terminal from which a third switching voltage for switching the VHF circuit unit into a mode of receiving a high-band television broadcasting signal is outputted; a transistor; and a resistor, wherein only the first switching voltage is generated and supplied to the VHF circuit unit and the first and fourth switching diodes are brought into conduction when the tuner unit receives the low-band television broadcasting signal in the VHF band television broadcasting signal or the FM broadcasting signal supplied from the first input terminal; only the second switching voltage is generated to apply a bias to the emitter of the transistor, and the second and third switching diodes are brought into conduction so as to supply the second switching voltage to the UHF circuit unit when the tuner unit receives the UHF band television broadcasting signal; only the third switching voltage is generated and applied to the VHF circuit unit and the first and fourth switching diodes are brought into conduction via the VHF circuit unit when the tuner unit receives the high-band television broadcasting signal in the VHF band television broadcasting signal; and both of the first and second switching voltages are generated, the first switching voltage is applied to the VHF circuit unit and the base of the transistor via the resistor, the second switching voltage is applied to the emitter of the transistor, and the second and third switching diodes are brought into conduction when the tuner unit receives the groundwave FM broadcasting signal Thus, by using the band switching voltage of the VHF circuit unit and the control voltage of the UHF circuit unit, the cable and the FM antenna connected to the antenna switch circuit can be switched.
According to a television tuner of the invention, switching means is provided between the demodulator and the pseudo carrier wave oscillator, the switching voltage generator is provided with a fourth output terminal for generating a fourth switching voltage for controlling the switching means when the tuner unit receives the FM broadcasting signal or the ground wave FM broadcasting signal, and the fourth switching voltage is applied to the switching means so that the pseudo carrier wave is supplied to the demodulator. Consequently, in association with the antenna switch circuit, the pseudo carrier wave outputted from the pseudo carrier wave oscillator can be supplied to the demodulator only when the FM broadcasting signal or the ground wave FM broadcasting signal is received.